Neuroscience proprioception

Proprioception= The ability to sense one’s body position in space

Systems Related to Proprioception

• Visual

How does vision influence proprioception? Environmental cues (seeing where you are and what's around you)

 What role does visual deficits play in a dysfunction of knowing where your body is in space? If you can’t see it is more difficult to know where your body is.

• Vestibular

 What roles does the vestibular system play in proprioception? Balance and equilibrium.

• Proprioceptive

Feedback/ feedforward loop between muscle spindles, Golgi tendon organs, joint receptors, and cerebellum. 

• Helps to perceive body position in space

Muscle spindle: maintenance of muscle tone (stretch reflex in biceps brachii)

• Two types of intrafusal fibers are located within the muscle spindle: nuclear bag fibers and nuclear chain fibers.

• Equatorial part: central region of a muscle spindle, which is a sensory receptor that detects muscle length and rate of change. (non-contractile/elastic)

• Polar part: the contractile ends of the intrafusal muscle fibers. Located at the ends of the spindle, which controls the tension and sensitivity of the spindle.

• Slow responding are tonic (static) and fast responding are phasic (dynamic) 

Muscle spindle Sensory Fibers

Type Ia (primary): Large; Heavily myelinated; fast conducting

Type II: Medium-sized; slow adapting; respond to changes in length 

• Activation: Muscle strength → Equatorial Region stretch→ Ia and II Sensory fibers 

Gamma Motor Neurons

• Gamma motor neurons: Play a key role in regulating the sensitivity of the stretch reflex by adjusting the tension in intrafusal fibers within the muscle spindle.

Gamma 1 (Dynamic)

• Have plate endings and terminate on the polar region of the nuclear bag

• Fast, rapid response 

Gamma 2 (static) 

• Have multi-branch endings and terminate on the nuclear chain

Golgi Tendon Organ (GTO)

• Embedded in tendons near muscle insertions

• Proprioceptors that detect tension in the tendon of a contracting muscle

• When activated, the GTO causes the muscle to relax (To protect the muscle from tearing)

• Use afferent sensory neurons

Activation: Agonist skeletal muscle contracts → Activate GTO→ Contracting muscle inhibited 

Joint Receptors

Function

• Respond to mechanical deformation at capsule and ligaments

• Provide proprioceptive information to the cerebellum and ventral horn

Sensory Stimulation

• Ruffini endings (II fibers)

• Paciniform corpuscles (II fibers)

• Ligament receptors (Ib fibers)

• Free nerve endings (A and C sensory fibers)

Joint Receptors and Proprioception

• Joint receptors work in conjunction with GTO and muscle spindles

• They are not critical to proprioception

Muscle Tone

Muscle tone: (the tension in a muscle at rest) Continuous state of muscle contraction at rest that helps to maintain a balance of opposing muscle groups around joint and postural control (Unconscious)

Muscle Strength: muscle’s ability to contract and create force in response to resistance. (Conscious) 

Neurological Mechanisms of MuscleTone

Upper Motor Neurons (Central Nervous System)

• Pyramidal/ medulla (Corticospinal tract)

• Basal Ganglia

• Extrapyramidal structures

-Brainstem centers (Vestibular and reticular nuclei)

-Vestibulospinal, rubrospinal, and reticulospinal tracts

     -     Cerebellum

-Feedback/ feedforward loops with above structures plus the muscle spindles, GTOs, & joint receptors

-Posterior and anterior spinocerebellar tracts, cuneocerebellar tracts, and rostral spinocerebellar tracts (Sensory/afferent) 

Lower Motor Neurons (Peripheral Nervous System)

• Motor neurons of ventral horn (Alpha and gamma motor neurons)

• Peripheral nerves that innervate skeletal muscles

Classifications of Muscle Tone

• Hypotonicity (low tone)

• Hypertonicity (spasticity)  (high tone)

  • Rigidity 

  • Clasp Knife Phenomenon

  • Cogwheel Rigidity

  • Lead Pipe Rigidity

  • Clonus 

Hypotonicity

• Abnormal decrease in muscle tone

• LMN lesions (motor neurons in ventral horn, peripheral spinal nerves, damage to posterior cerebellar lobes)

Hypertonicity (Upper motor neuron)

• Excessive muscle tone

• Spasticity: difficulty actively and passively moving muscles on one side of joint

• Rigidity: difficulty moving muscles on both sides of joint

• Clasp Knife: severe spasticity at a joint which will release with a sustained stretch

• Cogwheel Rigidity: increases muscle tone resulting in jerky, alternating movements

• Clonus: Uncontrolled oscillations (vibration/shaking) of a muscle

Etiology of Spasticity

• Hyperactive reflex arc

• Reduced reciprocal innervation of antagonists

• Loss of cortical or subcortical modification of Gamma Motor Neurons in Ventral Horn

• Damage to M1

• Damage to brainstem area that contain vestibular, reticular and pontine nuclei (extensor muscle tone)- decererbrate rigidity 

Therapeutic Techniques to Normalize Muscle Tone

Increase Tone (for hypotonicity)

• Quick stretch on agonist

• Tapping muscle belly

Decrease Tone (for hypotonicity)

• Slow, sustained stretch on agonist

• Deep pressure on tendon (activate GTO)

• Sprinting or serial casting